1. Field of the Invention
This invention relates to a method and apparatus for determining various properties of skin. More particularly, the invention relates to an in vitro method and apparatus for determining the viscoelastic properties and other characteristics of skin under various conditions.
2. Description of the Prior Art
In the course of determining the proper treatment of various skin conditions, it has been found necessary to evaluate the effects on the stratum corneum induced by various topically applied agents or other treatments. It is known that the various physical properties of the stratum corneum are related to its composition and macromolecular structure which are affected by its state of hydration or disease condition. Various techniques are known to measure the viscoelastic and dimensional properties of the stratum corneum and determine the changes in these properties which result under various conditions. These techniques include both in vivo measurements of various portions of the skin of living subjects and in vitro measurements of excised skin samples.
The usefulness of data derived from in vivo measurements is limited in view of the difficulty in making accurate measurements on living subjects. The measurement of viscoelastic properties requires extremely sensitive instrumentation capable of detecting small skin displacements. Because of the requisite sensitivity, this instrumentation also detects extraneous vibrations inherent in the use of living subjects and produces measurents having a relatively high experimental error. Furthermore, in vivo measurements are limited in their usefulness because of the inability to vary the ambient conditions to which the living subject may be exposed during the measurement process.
In vitro measurements of the viscoelastic properties of skin are somewhat more accurate since certain ambient conditions may be varied in a controlled manner and since the extraneous vibrations or other variations inherently introduced by living subjects may be essentially eliminated. One type of vitro technique is thermomechanical analysis (TMA) to measure changes in mechanical and dimensional properties of the stratum corneum as a result of temperature-induced transformations such as lipid melting and protein conformational changes. The TMA technique is well-known and involves the use of a weighted probe resting on a sample and the measurement of the probe's linear displacement as the sample is heated at a programmed rate. The procedure and some test results are described, for example, in an article written by William T. Humphries, M.S. and Richard H. Wildnauer, Ph.D. entitled Thermomechanical Analysis of Stratum Corneum, The Journal of Investigative Dermatology, 32-37, 1971.
Another technique utilizes a strain gauge to determine the dimensional changes of a skin sample immersed in a water bath of controlled temperature. Isometric Contraction of Epidermis and Stratum Corneum with Heating, Howard P. Baden, M.D. and Ann M. Gifford, The Journal of Investigative Dermatology, 298-303, 1970.
Both of the foregoing technique are limited in their ability to alter various other parameters affecting normal skin (e.g. humidity, temperature, topical agents, etc.) while simultaneously measuring the changes in the viscoelastic properties of the samples being tested. An example of a method and apparatus which is used for determining the viscoelastic properties of excised skin, and which overcomes some of the deficiencies of the foregoing techniques is described in an article entitled Viscoelastic Properties of Intact Human Skin: Instrumentation, Hydration Effects and the Contribution of the Statum Corneum, M. S. Christensen, C. W. Hargens III, S. Nacht, E. H. Gans, The Journal of Investigative Dermatology, 69: 282-286, 1977. The procedure entails the use of an electrodynamometer utilizing a gas suspended armature which oscillates in response to changes in the magnetic field generated by a surrounding coil. The coil is activated in a sinusoidal mode by a function generator, and the displacement of the armature is monitored by a linear variable differential transformer. A stiff wire probe connects the armature to the skin sample and measurements of the skin displacement are made as a function of the force applied to the armature. The time lag between the applied force and the resulting deformation of the skin surface is a characteristic of materials which, like skin, are viscous and not perfectly elastic. One disadvantage of this procedure is that it, like the previously described techniques, does not provide means to vary many of the ambient conditions under which the skin sample is tested. Topical agents may be applied, but the absence of a water bath will cause the sample to dry out over time, thus affecting the repeatability and reliability of any measurements being made. It should also be noted that the viscoelastic properties of the skin will be affected by the ambient temperature as well and this procedure does not provide means to vary this parameter.
In addition to skin's viscoelasticity characteristics, another parameter of interest is its transepidermal water loss under varying conditions. Known instrumentation for measuring this characteristic operates in vivo utilizing two spaced thin film capactors placed in an enclosure over a portion of a (human) subject's skin. The humidity gradient sensed by the two capacitors is calculated as necessary. An example of an instrument capable of performing this function in vivo is the Evaporimeter EP 1C or 1D, manufactured by ServoMed, Riddersviksva gen 107, Stockholm-Hasselby, Sweden. However, no instrumentation is known for measuring transepidermal water loss in vitro and under selectively variable ambient conditions.
While in vitro measurements of skin flexibility and other characteristics such as transepidermal water loss may be more accurate than in vivo measurements, it must be noted that, as explained above, prior art in vitro procedures suffer from the inability to adequately vary the temperature and humidity (and other) conditions to which living tissue is normally subjected. Data thus obtained with these previously known techniques is, therefore, of limited usefulness. There is, therefore, a need for an in vitro procedure and apparatus which can overcome the deficiencies of the prior art by enabling simulated in vivo measurements under conditions approximately those of living tissue.
It is accordingly an object of this invention to provide a method and apparatus capable of making accurate, simulated in vivo measurements of the viscoelastic properties of skin.
It is another object of this invention to provide a method and apparatus for determining the viscoelastic properties of skin samples in vitro while enabling control of ambient temperature and humidity to simulate in vivo conditions.
It is a further object of this invention to provide a method and apparatus for determining the viscoelastic properties of skin samples vitro under varying ambient conditions and varying topical agents.
It is still another object of this invention to provide a method and apparatus for determining the transedpidremal water loss of skin samples in vitro under varying ambient conditions and varying topical agents.